The method described here serves, in microscopes that work with reflection, for suitable formation of the illumination beam at a high light yield, and for adaptation of the illumination aperture to the entry pupil of the microscope lens. Using this invention, it is possible to create new possibilities of use for relatively low-intensity light sources. In particular, the use of LED light sources in microscopes that work confocally, which are considered to be very low-intensity, is made possible by means of this invention. The influence of internal light reflections is significantly reduced by means of suitable diaphragm positioning.